The software side seems a bit trickier. I use MythTv for my capturing, but it's sort of like using a blow-torch to solder a resistor on a circuit board -- it can be done, but it's really not the right tool. :-)

VLC is probably your best best for what you want to do. It can stream data from the PVR-150 to a file. You will want to have a look at this page on capturing with the PVR-150 to understand how to choose the input source on the PVR-150.